1. Field of the Invention
This invention directs itself to a system and method for determining the volatile constituents and vapor pressure of a liquid whose composition is unknown. In particular, this invention directs itself to a portable test set-up which can be utilized on-line, or configured to duplicate actual process conditions. Still further, this invention directs itself to a test system wherein the fluid to be tested is passed through a gas/liquid separator at a predetermined temperature and under predetermined pressure conditions. The physical properties of liquid exiting the separator is measured and the liquid passed through a flow meter while the gas evolved therefrom is coupled to a gas meter and a gas chromatograph. More in particular, the liquid flow rate, liquid density and molecular weight, the gas flow rate, the gas composition data obtained from the gas chromatograph, as well as the temperature and pressure data are all input to a processor which performs an iterative calculation to compute the composition of the liquid being tested, and from the derived liquid composition, the vapor pressure is then calculated. From the derived composition, the quantities of vapors and other environmental emissions can also be calculated supposing the fluid were to be heated to a temperature at which its vapor pressure exceeds atmospheric pressure and therefore boiling occurs. Further, this invention directs itself to a method of converging the complex multi-component equilibrium calculation data with the measured physical parameter values to derive the composition of the liquid being tested, such liquid composition being determined to produce a gas composition and flow rates which substantially equal the actual measured values.
2. Prior Art
Test methods for determining vapor pressure and related calculations are known in the art. Vapor pressure is important to understand in view of the potential phase changes that can occur with decreases in fluid pressure or increases in fluid temperature. The text "Chemical Principles", published by W. B. Sanders Company (1969) disclosed on Page 247 that " . . . a liquid boils at a temperature at which its vapor pressure becomes equal to the pressure above its surface . . . ". This vapor pressure definition is the same for both pure components and mixtures of different components such as oil. For example, water boils at 212.degree. F. at atmospheric pressure (14.7 psia); yet if placed in a system at 7.5psia the water will boil at 180.degree. F. since the vapor pressure of water is 14.7 psia at 212.degree. F. and 7.5 psia at 180.degree. F. Likewise, if the oil has a vapor pressure greater than atmospheric it will boil when fed to an atmospheric tank, with the gases evolved becoming a consideration in terms of safety, environmental emissions, and even operations of tanks, pumps, and metering equipment.
The best prior art known to the Applicants include U.S. Pat. Nos. 4,395,503; 4,459,266; 4,460,544; 4,522,056; 4,667,508; 4,799,166; 4,901,559; 5,020,000; 5,301,125; 5,305,231; 5,327,779; 4,783,989; and, 5,172,586.
In some prior art systems, such as that disclosed by U.S. Pat. No. 4,395,903, vapor pressure is determined by off-line, laboratory methods. In such systems, a sample of the liquid is maintained at a predetermined temperature and pressure, while a sample of the hydrocarbon gas mixture is analyzed utilizing a gas chromatograph. The vapor pressure of the mixture is calculated by summing the partial pressures for each of the components of the gas which were determined by the chromatograph. This method fails to consider, however, the relative volume of gas to the relative volume of liquid, which is an important parameter for determining vapor pressure.
In still other systems, such as that disclosed by U.S. Pat. Nos. 4,783,989 and 5,172,586, the vapor pressure of a liquid composition is measured utilizing a sample which is disposed within a cylinder apparatus having a displaceable piston, permitting the chamber containing the liquid under test to be expanded and the resultant change in pressure therein measured and utilized to plot equilibrium pressures versus chamber size, which are subsequently extrapolated to determine the pressure at a chamber size of minimum expansion which approximates the vapor pressure.
In the standardized petroleum industry test, it is very common to determine vapor pressure utilizing the Reid vapor pressure test (ASTM D-323-90) in combination with a nomograph (American Petroleum Institute 2517) for calculating the "True" vapor pressure at a predetermined temperature. However, that "True" vapor pressure process leads to errors on the order of 50%-300% when compared to the system and method of the present invention. In the standardized Reid test, a liquid sample is collected at atmospheric pressure. This introduces the first source of error since the high vapor pressure components escape. The higher the temperature of the sample being collected, the greater this error becomes. The test then proceeds by chilling the sample in an ice bath to which a volume of air, heated to 100.degree. F., is coupled to the liquid containing chamber, the air containing chamber having four times the volume of the liquid containing chamber. The air and liquid constituents are then shaken and put in a bath at 100.degree. F. The pressure within the container is then measured to establish a vapor pressure value. The nomograph is subsequently utilized for adjusting the Reid to estimate the "True" vapor pressure at other temperatures. Since the sample of liquid is of a predetermined volume those constituents which enter the air chamber's gas phase are also lost from the liquid phase by contact mixing with the air, creating another source of error. In addition, the air chamber acts as an expansion damper which further decreases the measurement value of the liquid's vapor pressure. Thus, both the former and latter sources of error contribute to the Reid test value yielding a lower vapor pressure result which even after adjustment by the API 2517 nomograph is lower than that which will actually exist at a predetermined temperature.
Therefore, it is an object of the present invention to provide a system and method for more accurately predicting the vapor pressure for a liquid, and in particular, it is well suited for determining such for complex multicomponent hydrocarbon compositions such as crude oil.
In addition, the method of the present invention determines the volatile components of the liquid from which the vapor emissions of the liquid can be calculated if the vapor pressure exceeds atmospheric at a given storage tank temperature.